FIG. 1 is a midway process of manufacture of a duct, in which a main duct 1 is formed into a rectangular tube by combining edges 2 of four surface treated steel plates by means of caulking or crimping, an L-shaped corner piece 5 shown in FIG. 2 is fitted to a corner 4 between adjoining flanges 3 and 3 in order to fill the corner 4, and a portion 6a of a bent piece 6 at tip end of the flange 3 corresponding to the corner piece 5 is bent over the corner piece 5 so as to secure the corner piece 5 as illustrated by FIG. 3. Conventionally, in order to bend the portion 6a, a method to tap or hit the portion 6a with a hammer has been employed. In this case, however, not only skill and labor are required but a large noise is emitted.
An object of this invention is to enable a caulking work without requiring skill and labor and, in addition, quietly.